The present invention relates to methods for machining steel blank materials to form steel extrusion dies, and more particularly to a modified chemical drilling process which improves the performance of such extrusion dies for the extrusion of plasticized inorganic powder batch materials.
The manufacture of inorganic honeycomb structures from plasticized powder batches comprising inorganic powders dispersed in appropriate binders is well known. U.S. Pat. Nos. 3,790,654, 3,885,977, and 3,905,743 describe dies, processes and compositions for such manufacture, while U.S. Pat. Nos. 4,992,233 and 5,011,529 describe honeycombs of similar cellular structure extruded from batches incorporating metal powders.
The manufacture of extrusion dies for the production of ceramic honeycombs by these methods requires extremely precise machining. To supply material to the slotted honeycomb discharge section of such a die, the inlet or batch supply face of the die is provided with multiple apertures or feedholes through which the plasticized material to be extruded is forced under high pressure. For softer steels, the feedhole array can be formed by mechanical drilling, but if the die is to be formed of harder materials such as stainless steels, electrochemical machining techniques offer better results.
In the electrochemical machining (ECM) process, also referred to as the STEM (Shaped Tube Electrolyte Machining) process, the apertures are formed through a controlled deplating (dissolution) of the electrically conductive steel workpiece. An electrolytic cell is formed wherein the drill comprises the negatively charged electrode (cathode), the workpiece comprises the positively charged electrode (anode), and the electrolyte is a flowing electrically conductive fluid.
In the manufacture of extrusion dies the drill (cathode) suitably comprises a metal tube of titanium having an insulated outer surface extending toward an exposed tip. This inhibits electrolytic action everywhere except at the tip of the tube. The workpiece is typically a high grade stainless steel plate or block, and the electrically conductive electrolyte fluid is nitric acid.
To drive the electrolytic dissolution of steel proximate to the tip of the shaped tube to form an aperture, an electric potential is applied between the tube and the anodic workpiece. As drilling proceeds, the electrolyte is continuously pumped through the drilling tube and into contact with the steel workpiece, thereby completing the electrolytic cell while flushing dissolved steel away from the workpiece surface.
In this "forward cycle" phase of the drilling process, the electric potential is typically controlled to a constant voltage or, more preferably, to a constant current, and the drilling rate or tube feedrate is controlled to a constant value. The purpose of these controls is to maintain as much as possible a constant aperture and an optimally smooth aperture surface finish. U.S. Pat. No. 4,687,563 to Hayes, U.S. Pat. Nos. 5,320,721 and 5,322,599 to Peters, U.S. Pat. No. 5,507,925 to Brew, and published EP Application No. EP 0 245 545 describe the ECM process as it has been applied to the fabrication of steel extrusion dies, and variations on those processes.
Although ECM remains the process of choice for forming large arrays of fine apertures in tools such as extrusion dies, problems with this process still remain. One difficulty relates to small variations in aperture surface finish, presently attributed to random variations in processing factors such as electrolyte temperature, workpiece metallurgy, electrical contact, and a myriad of other process variables or their interactions.
In the case of honeycomb extrusion dies, one effect of such variations is that surface finish can vary from aperture to aperture across the aperture array forming the inlet to the die. This can result in an uneven flow front in the material being extruded from the outlet or discharge face of the die, a condition which can produce a variety of honeycomb product defects, including but not limited to significant variations in cell dimensions, cell shapes, and cell wall thicknesses in the honeycomb structures being extruded.
In fact, feed-hole finish appears to be a fundamental factor influencing the flow characteristics of honeycomb extrusion dies which is not yet within the ability of the art to completely control. This inability to control surface finish means that the extrusion process may not be operating in a processing regime that is optimal for high quality honeycomb production. If better surface finish control were possible, engineered surface finishes providing significant improvements in extruded product quality might be attainable.